Cover panel for an expandable container to provide headspace control in a food can

Abstract

 The invention suggests a peelable cover panel for a lid of peel type which allows a safe sterilization of filled and closed cans in continuous autoclaves (high temperature prevailing there and a resultant high pressure in the can) avoiding risk of breaking, breaking up or detaching said plane (surface oriented) cover panel on or from the seamable annular ring. After the cooling of the can, the lid also has an acceptable appearance. Suggested therefore is a cover panel for sealing to an annular ring to provide a lid to close a can body and allow a thermally treating pasteurization or sterilization process of the closed can in a continuous autoclave. The cover panel (3) having a radially outer ring band (3a) for sealingly affixing to a flat web (6) and a remaining central portion (3b) that is surrounded by the ring band, the control portion being preformed in a bowl-shaped fashion and is for this reason stabilized. The stabilization is such that the bowl-shaped cover panel (3), during an increase in pressure during at least the pasteurization, changes from the preformed bowl-shape as first position (3b) to a second position (3b') that is bowl-shaped axially outwards, and is still able to return to the first position (3b), at least substantially during or after cooling.

Description

[0001] The invention relates to cover panels for lids for cans for receiving foodstuff, which are subject to a thermal treatment above 50°C in the form of sterilization or at least pasteurization. Methods for the production of the lids (claim 20) and for securing the leak tightness of the sealing (claim 8) are also covered.

[0002] Lids are concerned having an annular ring for seaming to the can body, the lid can be firmly and tightly connected with the can body rim and is in particular made from metal, and a "lid diaphragm" (diaphragm or panel) in the form of a cover surface which is affixed to the annular ring (for seaming to the can body) in such a way that, for the opening of the can, the panel can be pulled off from the seamable annular ring or may be peeled off from it by means of pulling. This relates on the one hand to peeling foils and, on the other hand, also to foils which are to be torn at the edge or are to be further torn.

[0003] One design of such a lid cover is known from US-A 4,211,338 (Bublitz). Difficulties arise at high temperatures, at any rate above 50°C, as they are - as a rule - present in sterilization or pasteurization processes in continuous autoclaves (retorts). The lid will leak or the lid diaphragm even begins to detach itself from the annular ring under the action of a difference in pressure which acts upon the lid, or can be damaged in such a way that this would result in a later leakage at the sealing line.

[0004] Consequently, these cans are mostly sterilized in such autoclaves (batch retort stations) which are equipped with means for generating an outer air or vapor pressure in order to apply a sufficient counter-pressure onto an outer surface of the lid, opposing the internal pressure in the closed can, due to which the lid, in particular the lid diaphragm, will be protected against too high differences in pressure ΔP.

[0005] It is not possible (or it requires great effort) to equip "continuous autoclaves" with such counter-pressure means that are suitable for continuous passage.

[0006] Continuous autoclaves (for a pasteurization or a sterilization process) with a counter-pressure due to a vapour atmosphere certainly generate a low permanent counter-pressure on the lid surface of up to 1.6 bar (0.16 MPa), but they are not sufficient for stabilizing customary "peel lids" without damage.

[0007] In addition to this, the lid surface would not form a smooth, fine-looking surface after cooling of the cans, which impairs acceptance by the customers and results in illegibility of any type of inscriptions or bar codes by scanners.

[0008] WO 2005/005277 A1 (Crown) discloses a lid that has a lid ring and a central panel which includes at least one concentric bead. The lid ring has an inclined flat web that is tilted axially upwards. Onto this slanted seal, that is called bonding panel in said document, the diaphragm as central foil is sealed. This diaphragm is deflectable outwards and gives the container an increase in volume, when subject to a pressure differential, as is the case during thermal processing. As said diaphragm has stretch introduced prior to processing (by the provision of said beaded profile), this beaded profile is deflected and serves for a generally domed shape after thermal treatment, page 2, bottom paragraph, page 3, first paragraph. The pre-stretching therefore concerns said at least one concentric bead (beaded profile) in the diaphragm, and pressure control takes place (page 4, second paragraph).

[0009] It is an object of the invention, to suggest a peelable cover panel for a lid of peel type which allows a safe sterilization of filled and closed cans in continuous autoclaves (high temperature prevailing there and a resultant high pressure in the can) avoiding risk of breaking, breaking up or detaching said plane (surface oriented) cover panel on or from the seamable annular ring. After the cooling of the can, the lid should also have an acceptable appearance.

[0010] These objects are achieved according to claim 1 (cover panel) or claim 20 concerning the production process of the lid.

[0011] The invention comprises steps for "ensuring" the tightness of the can closures during sterilization in a continuous autoclave in the sense of a securing or providing of tightness on the can closures that work with an "annular ring" (for seaming to the can body).

[0012] A production process for the can closing is separately claimed (claim25).

[0013] The invention is not limited to lids for cans of a circular shape, but can also be applied with the same effect and the same advantages to cans of other circumferential or cross-sectional shapes such as oval, rectangular, rectangular with rounded corners or square shapes of cans (can bodies).

[0014] The "dome-shaped or bowl-shaped preforming" of the cover panel allows - after filling with food or foodstuff and upon closing of each can with the lid - to substantially reduce the headspace in the can. The curved shape with its centre may project substantially downwards beyond that area of the annular ring that is placed innermost of a can body inside. If, under the internal pressure formed during the thermal treatment in the interior of the can, the dome-shaped or bowl-shaped cover panel portion changes, in particular abruptly (suddenly) changes, to a bulging position that is mirror-inverted to the original position, now towards the outside (claim 1), resulting in a substantial enlargement of the headspace and thus a reduction of the pressure in the can formed during thermal treatment.

[0015] This change function is supported by a stiffening of the cover panel at least in the central portion by the preformation of this portion. A plane material is used for this purpose, which itself or a layer thereof will get harder due to the preferred deep-drawing process. Thus, a certain shape keeping hardness or stability results from this (claim 2) as a tightening. Said stability is distributed across the entire panel surface, radially within the annular ring. The panel surface is named plane or "areal oriented" as the surface has a lateral extension but on a curved panel, that is plane, but not flat.

[0016] This stability achieves that practically the same, but reversed or inverted panel shape is obtained upon outwards directed bulging. It corresponds to the inverted original dome shape without increasing the surface area, without plastic preformation, in particular without a "stretching" of the panel. After the temperature is reduced, the central portion of the cover panel is again returned to its preformed original shape upon cooling, which it adopts without any further help (due to the vacuum formed in the inner space and "under the cover panel").

[0017] The smooth dome or bowl shape (preformed bulging) of the lid of the can that is ready for sale is very fine-looking and does not encounter any problems as regards acceptance by customers.

[0018] The process is for producing a lid closure for cans containing foodstuff (claim 20). The lid provides tightness during sterilization or pasteurization in a continuous autoclave. Said cans being closed with a can closure as lid. The can closure is produced from an annular ring (adapted for seaming to the can body, a so called "Deckelring") and a lid panel having an outer ring band. The band is sealingly placed onto an inner flat web. Prior to attaching the lid to the filled can a central area of the lid panel is reshaped by preferably deep drawing to a smooth bowl shape or a dome shape with a plane surface, this as "original shape". An outer ring band limits the central area (surrounds it in case of a circular lid). A material of this central area is solidified or hardened by the deep drawing process (step), this to such an extent that under an increased pressure in a headspace of the can during the can's passage through an autoclave station, the central area changes to an axially outwardly bulging shape that is mirror-inverted. The inversion is with respect to the "original shape". During a subsequent cooling of the closed can, the central area automatically returns to the "original shape". This is at least substantially the same (claim 1).

[0019] The suggested cover panel permits a sterilization or pasteurization of the filled cans at the pertinent high temperatures and differential pressures in continuous autoclaves readily and without any risks, i.e. without any measure for generating a counter-pressure that additionally acts from the outside (other than the steam pressure). The steam pressure (vapor pressure) is regularly present, higher than atmospheric pressure, but not high enough to support forces on the outer surface of the lid.

[0020] The dimensions of the preformed shape (of the central portion) can be adjusted to the diameter and the volume of the cans. Likewise, the inclination of the flat web of the annular ring for seaming to the can body to which the outer ring band of the panel is affixed is adjusted in such a way with respect to a horizontal plane that an imaginary extension of the surface of the flat web extends at best tangentially to the dome- or inverted-bowl shaped central portion that bulges outwardly under pressure (claim 5). The change of shape occurs preferably on an inner pressure level of 0.15 MPa (claim 4).

[0021] The inclination of the web is directed upwards; this is "outwards axially" when the can body is taken as a reference, that is closed by the lid panel and the lid ring.

[0022] A preferred lid is adapted for closing a can with a diameter of 83 mm (claims 7, 10). The depth of the preformed lid panel is between 5 mm and 6 mm, approx. 5,6 mm, the lowermost point of it being about 3 mm below the lowermost points of the annular ring for seaming to the can body. The bulging corresponds to a sphere segment in the case of a circular cross-section of the lid. The angle of the flat web is preferably between 22° and 25° with respect to the horizontal (claim 6). Here, peeling forces are practically completely avoided (claim 5).

[0023] The smooth/plane bowl/dome surface of the preformed lid panel is not disturbed by or interfered with any undulations or grooves (claim 10).

[0024] The filled can with the lid can be at least pasteurized, in particular even sterilized, in practically any of the known continuous autoclaves without additional counter-pressure means. The food in kept therein for a long time, resulting from thermal treatment

[0025] The production process of the lid closure is the subject matter of claims 20. The preformation (preforming) of the panel in the central area takes place in the same fashion. The processes permit the use of already used machines, in particular during a sealing on a plane flat web with subsequent inclined deformation of the web upwards/outwards (claim 22). The panel surface may be applied onto the already inclined flat web or the - still plane - flat web which is to be inclined after heat sealing.

[0026] The invention is explained in greater detail in the following, using schematic drawings and embodiments serving as examples.

Fig. 1

shows a section through a lid according to one example of the invention.

Fig. 2

shows the transition area between annular ring for seaming to the can body and lid diaphragm on a larger scale.

Fig. 3

shows a schematic sectional representation of a concrete example of a can with a predetermined diameter.

Fig. 4

shows a representation of a concrete example of a preformed lid panel.

Fig. 5

shows a lateral view of a representation of the concrete example.

Fig. 6

2D sphere shape of a panel on a can body in theoretical evaluation (no annular ring displayed).

Fig. 6a

is a 2D sketch of Figure 6.

Fig. 7

shows the 3D model of Figure 6a.

Fig. 7a

is a 3D representation for explanation of force and tensile stress.

[0027] As can be seen from Figure 1, a lid 1 has an outer annular ring 2 suitable for seaming to a can body and a lid panel 3.

[0028] The annular ring 2 (for seaming to the can body, in short: "lid ring", seaming ring or "annular ring") is e.g. made of sheet metal. It comprises an outer rim portion 4 as a "flared flange" for firm and liquid tight connection with a rim 24 of an opening of the can body, cf. Fig. 3. The flange 4 is connected through a core wall 5 with the flat web 6 projecting generally radially inwards. The flat web 6 that extends all around is axially outwardly inclined or tilted at an angle larger than zero with respect to a horizontal plane that extends perpendicular to a vertical axis 8 of the lid 1. The radially inner edge of the flat web 6 is axially bent towards the inside and is designed in a sterile fashion, in particular by means of a curling 7. It may also be replaced with a relatively blunt inner edge. In the case of other can shapes, the lid shapes and the "annular ring" for seaming to the can body are accordingly adapted (seen in the horizontal direction).

[0029] The lid panel 3 comprises an outer continuous ring band 3a which at least partially covers the flat web 6 of the annular ring from the outside, if the panel 3 is tightly connected with the annular ring 2 in a fastening strip 13, e.g. by means of contact sealing or induction sealing (pressure sealing, ultrasonic sealing, laser sealing). This ring band 3a limits the central portion 3b (provides its outer limit), a transition portion 3c between the two being within the area of the curling 7 after connection of the panel with the annular ring.

[0030] The central cover portion 3b of the lid panel is preformed by a deep-drawing process. This process can be implemented prior to or after the connection of cover panel 3 and annular ring 2 in the fastening strip 13. The stabilizing reshaping only covers the central portion. It is implemented across the entire surface of the panel.

[0031] The preforming converts the central portion 3b into a dome or bowl shape in the axially inwards direction, the edge of the bowl is in the transition portion 3c to the outer ring band 3a and the lowermost centre 3d of the panel is clearly, in particular at least a few millimeters below a horizontal plane 18 that passes through the lowermost points of the annular ring 2 according to Fig. 2. This also corresponds to the plane E2 of Fig. 3 in the example.

[0032] It is advantageous if, as is preferred, the panel itself comprises a material that is stiffened or hardened by the deep-drawing process such as aluminum or the like, or contains at least such a layer. Due to this, the preformed central portion 3b is provided with an inner shape (or dimensional) stability. This is of advantage for the entire appearance of the finished, closed and thermally treated can package.

[0033] During sterilization, if temperature and consequently pressure are generated in the interior of the filled can that was closed with the lid (cf. also Fig. 3), the preformed central portion will change, in particular change abruptly, from its "die sunk", undulation-free dome/bowl shape to a practically mirror-inverted shape that is axially outwardly bulging (sphere shaped in the case of round cans) as it is outlined in a dash-dotted fashion at 3b' in Fig. 1 and Fig. 3.

[0034] Due to the stiffening or hardening of material of the cover panel achieved during the preforming, the dilatability of the central portion 3b is accordingly practically zero so that even in the case of high pressures formed in the can during (thermal) treatment in a continuous autoclave, the outwardly bulging shape of the central portion can be determined in advance (by means of a computing).

2-D Model.

[0035] Figure 6 is a can with domed panel (pre shaped membrane) under internal pressure and cross section in Figure 6a of the convex shaped panel. Figures 6 and 6a show the two-dimensional representation of a can with a domed panel under internal over-pressure. The two-dimensional model of the pre-shaped domed panel shows its convex shape under internal over-pressure P. The parameters which indicate the geometry are given in the figures.

[0036] D is the diameter of the inner radius of the sealed zone, which is different from the can diameter, h is the dome deflection, y and z are the axes indicators, α is the angle of the dome with the y-axes. The volume increase, the angle and the radius of the convex dome can be calculated with the following equations:

3-D Model.

[0037] Figure 7 is a convex shaped domed panel in 3-D coordinate system having x, y and z; ϕ, Θ (Theta) and p (rho). Figure 7a displays a force F on a randomly chosen small part (segment) of the domed panel and a part cross-section of it.

Stress in domed panel.

[0038] The tensile stress in the domed panel can be calculated rather straight forward with Figure 7a using the force known from the multiplication of the pressure and the surface segment. The force has to be divided over the length of the side (I) and the thickness (e) of the domed panel.

[0039] The tensile stresses on all sides are therefore given by

[0040] The radius of curvature of the domed panel can be expressed in can dimension parameters. The tensile stress in the domed panel is

[0041] In this equation ...
P is the pressure [N/mm²]
p is the radius of the convex shaped domed panel
e is the domed panel thickness
h is the deflection of the domed panel
D is the inner radius of the sealed zone.

Slanted sealed zone (Sealing strip).

[0042] The sealed zone can be "bended up" in such a way that the sealed zone is slanted upwards (axially outward, or upwards) and parallel to the domed panel. In this situation there is only a shear stress in the sealed zone and no longer a peel stress. The following relation between the tensile stress in the panel and the shear stress in the sealed zone applies

[0043] In this case the shear stress can be calculated by

[0044] As outlined in Figure 1, the depth 10a of the bowl shape and the depth 10b of the bulging are practically equal. Referred to the plane 15, the volume in the depth portion (defined by 10a) is equal to the volume in the depth portion (defined by 10b). The depths/distances of the centers of the deformed lid panel represent the volume formed vis-à-vis the central plane 15 or E3. Upon the closing of a can body with the lid 1, the headspace H of the can is reduced by the dome/bowl volume (between 15 and 3b) and, upon heating during sterilization, the volume of the headspace is enlarged by the total volume 12 (from depths 10a and 10b).

[0045] Both contribute to a clear reduction of the maximally occurring pressure and secure the closed cans against damage upon their passage through the autoclave. Pressures of less than 1 bar (0.1 MPa) can be achieved, which, without the preforming of the panel would be clearly above this value, e.g. at 1.5 bar (0.15 MPa). This amount of the achieved lowering of the pressure depends in general on the temperature of the foodstuff filled in. At hot filling of food the differential pressure that occurs as a maximum on the panel is lower than the differential pressure when using cold filling such as for pet food as "foodstuff".

[0046] The stability as to shape/form, i.e. the avoided permanent deformation (as a missing plastic deformation or - at most - a residual elastic deformation by means of the modulus of elasticity) of the central portion 3b contributes to the fact that, upon the cooling of the finished sterilized can, this preformed portion 3b practically exactly re-adopts the original dome/bowl shape. In both conditions or positions or according to panel shape 3b and 3b' no undulations are contained in the panel. The bowls or domes are smooth (also called bulged, but with a plane surface in the bulging).

[0047] The fact that the central portion retains its area (in an envelope) permits the advance calculation of the measure of its bulging in the case of the pressures to be expected as a maximum during sterilization so that the angle of inclination 11 of the flat web 6 of the annular ring 2 for seaming to the can body can be adjusted to this right from the beginning. By no means is the angle smaller than the angle of a tangent at the bulging of the central portion 3b (next to the slanted web). The angle 11 is rather selected larger with preference so that that - in the case of the maximum internal pressures formed in the continuous autoclaves - practically exclusively shear forces and no peeling forces are active as resultant forces in the ring band 3a of the panel 3 that is affixed to the flat web.

[0048] The angle 11 is set to more than 20°. The radius or the transverse dimension (in the case of a deviation from the circular shape) of the central portion is shown as 9. Reference 16 in Figure 2 emphasizes that the portion 3b in its bowl shape projects down to clearly below the plane 18 which passes through the lowermost portions (or points) of the annular ring 2.

[0049] The dimensions of the preforming and that of the angle of inclination depend upon the volume and the radial dimensions of the can and thus also on the size of the lid. The smaller the radius of the bulging in a pressure-loaded condition is, the smaller is the mechanical stress in the lid panel.

[0050] A suitable material of the lid panel 3 is a thin metal, preferably an aluminum, which is used for the body diameter of 83 mm. Other diameters may be used in the following manner, in a range of diameters between substantially 50 mm and 100 mm (for Europe), in particular with especially customary diameters: 73 mm, 99 mm, 65 mm, 83 mm; similar for containers (bodies) made of steel sheet.

[0051] The can body may be made of aluminum or steel sheet that are covered with a varnish.

[0052] The annular ring 2 is preferably made from an aluminum covered with varnish, the outer varnish layer being a hot sealable sealing layer, which is sealingly connected with the annular ring in the sealing portion 13. Instead of metal the ring material may also be plastic material or a plastic/metal composite, e.g. produced by means of an injection process with or without an insert or with a previous inserting of the lid panel in the shaped opening for the ring. Annular rings made of steel can likewise be used.

[0053] Instead of the hot sealable layer on the ring, ring 2 may also be laminated or extruded with polymers. The lamination of the ring is done prior to the cutting out and the shaping of the annular ring.

[0054] In a preferred embodiment the lid panel that is connected with the annular ring (for seaming to the can body) preferably comprises several layers:

coating varnish layer

print layer

aluminum layer (about 70 µm, in the range of 30 µm to 100 µm)

extruded polymer layer (material with approx. 12 g/m² to 30 g/m²)

[0055] The extruded polymer layer is a co-extruded layer of a tie layer and a peel layer. Other extrusions and laminations can likewise be used.

[0056] The lid panel 3 was reshaped (deep-drawn) to a convex shape in the central area 3b as it is shown in Fig. 1 at 3b. In the example, the convex shape 3b has a radius of 110 mm. The lid layer was sealingly affixed to an initially horizontal flat strip, in a connection area 13 which is at first not upwardly inclined. The flat strip 6 of the ring 2, which supports the connection area was then upwardly deformed in order to obtain the inclination position of the angle 11 of about 24°, measured with respect to a horizontal plane 18/E2. This applies to the diameter of 83 mm of can and ring.

[0057] The sealing of the ring band 3a of the lid panel 3 can be achieved more easily with a horizontal flat web 6 than with an already inclined flat web. Consequently, the lid panel 3 may still not have any preforming shape of its own, but will only be provided with a corresponding preformed shape after the sealing in the connection area 13 as sealing strip. Here, the central area 3b is preformed to a bowl shape by means of the reshaping and stiffened or hardened, in order to admit hardly any elastic deformation, but to be capable of changing to a practically mirror-inverted, outwardly bulged bowl/dome shape in the case of an inner excess pressure. The central area is lowered that much below the plane 18 that there are several millimeters between the lowermost point of the initial bowl shape 3b and this plane (in the preformed state).

[0058] After the reshaping of the central area 3b an upwardly directed reshaping of the flat strip 6 (or the web) can be carried out. This obtains its inclination of more than 20° in this connection.

[0059] In a preferred embodiment that is not depicted these two re-shapings, that of the bowl-shaped bulging of the lid panel with a hardening, stiffening character and that of providing an inclination of an annular part the annular ring for seaming to the body, may also be carried out practically at the same time.

[0060] In the example there was a sealing strip 13 as the connection area of the still not preformed lid panel 3 on the initially horizontally oriented flat strip 6 of the ring, which was produced by implementing a sealing with the following parameters

ϑ = 190°C	temperature
P = 150 kg	Pressure
t = 300 msec	Sealing time.

[0061] The inwardly bulged bowl/dome shape had - as represented above - a maximum deflection as the depth 10a after the upwardly directed inclination of the flat web 6 which depth was between 5 mm and 6 mm, with a mean value of about 5.6 mm within a probe of a plurality of tests.

[0062] Figure 3 elucidates again the important advantages of the lid that is capable of expansion.

[0063] The lid 23 is shown in its position after firmly and tightly fixed to a can body 20, which is filled with the foodstuff 21 and then closed. A symbolic filling height is outlined at 22 or the level E1, above which the headspace H filled with air or vapor is located. The axis of the can is designated 25. The annular ring (for seaming to the can body) and the can body rim are connected with each other in customary fashion by means of a double seam 24a at the end 24 of the container (represented in a seamed fashion on the lefthand side, and in a placed fashion on the right-hand side in Fig. 3). The connection area between the flat web of the annular ring and the ring band of the lid panel is designated 26. The central portion 27a is deep-drawn in a dome/bowl-shaped fashion. Its depth 30 is represented exaggerated in order to display that it clearly reaches below the lowermost portion (plane E2) of the annular ring for seaming to the can body. The bowl volume defined by its depth 30 reduces the headspace H by the same volume, whereas the volume allocated to the double arrow 31 and limited by the central portion in its concave dome/bowl and convex bulging shapes outlines the volume enlargement of the headspace H with maximum pressure load ΔP during thermal sterilization. The broken line extension 28 of the flat web makes it clear that the angle of the flat web is larger than the angle 11 of the tangent to the bulging 27b.

[0064] A lid for a can with a diameter of 83 mm is assumed as a further practical example. The dome/bowl depth 10a/30 of the preformed lid panel is between 5 mm and 6 mm, about 5.6 mm, the lowermost point 30d of the bowl being about 3 mm below the lowermost points of the annular ring. The bulging corresponds to a sphere portion - in the case of a circular cross-section of the lid as is shown by Figures 4 and 5. The angle 11 is between 22° and 25°. Here, peeling forces are practically completely avoided.

[0065] The reference symbols in Figures 4 and 5 are consistent with the ones used before. Figure 4 additionally shows a tab to pulling off the panel 3 (having sealed ring band 3b and central panel 3a).

[0066] The smooth/plane dome/bowl surface is not disturbed by any undulations or grooves. The can may be at least pasteurized, in particular sterilized with the lid in practically each of the known continuous autoclaves without counter-pressure means.

Claims

1. Cover panel for sealing to an annular ring to provide a lid to close a can body and allow a thermally treating pasteurization or sterilization process of the closed can in a continuous autoclave,

- the cover panel (3) having a radially outer ring band (3a) for sealingly affixing to a flat web (6) and a remaining central portion (3b) that is surrounded by the ring band, the central portion being preformed

- axially towards an interior (of the can) - in a bowl-shaped fashion and is for this reason stabilized in a manner

- - so that the bowl-shaped cover panel (3), during an increase in pressure during at least the pasteurization, changes from the preformed bowl-shape as first position (3b) to a second position (3b') that is bowl-shaped axially outwards, and is still able to return to the first position (3b), at least substantially during or after cooling.

2. Cover panel of claim 1, sealed to an annular ring to provide a lid, said lid (1) having the annular ring (2) for seaming to a can body edge portion and a plane or surface oriented cover panel (3;3a,3b) disposed sealingly (13) on the annular ring to the ring (2), wherein the annular ring comprises the flat web (6) that extends towards a vertical central axis (8) of the lid and is inclined at an angle (11) with respect to a horizontal plane (15), the inclination is oriented upwards or axially outward of the can body.

3. Cover panel of claim 2, wherein the annular ring comprises the flat web (6) that is inclined upwards or outwards with respect to a horizontal plane (15) at an angle (11) of more than 10°, to which inclined web the cover panel (3) is seal ingly affixed by virtue of the radially outer ring band (3a), wherein the lid panel comprises a material that can be hardened or stiffened by a deep-drawing deformation or comprises a layer of such a material, and wherein a central portion (3b) surrounded by the ring band (3a) has a first shape continuously bulged due to the deep-drawing process providing only such shape keeping hardness or stability, to enable this central portion (3b)

- to change into an outwardly shaped bulging (3b') under an inner pressure occurring during the temperature treatment inside the closed can, and

- upon reduction of said pressure to return this portion (3b) to the first shape of continuous bulging.

4. Cover panel according to claim 1 or 2, in which - when referred to a horizontal plane (15) assumed to pass through a transition area (3c) between the outer ring band (3a) and the inner central portion (3b) - a first distance (10b) of a center of the central portion (3b) from the horizontal plane (15) in said outwardly bulged position of the central portion is substantially the same as a second distance (10a) in said bowl-shaped shape (3b).

5. Cover panel according to any of the preceding claims, in which an axial distance (10a,10b) is dimensioned in such a way that, due to the shape change of the central portion (3b) from the first position to the second position upon increase in pressure of preferably 1.5 bar (0.15 MPa), an increase in volume of a headspace caused in the closed can (1) limits a maximum pressure to a value not being detrimental to the lid during one of sterilization and pasteurization of the closed can in a continuous autoclave.

6. Cover panel according claim 2, wherein the angle of inclination (11) of the flat web (6) is coordinated with the distance (10a,10b) of the center of the central portion (3b) from a horizontal plane (15, E3), which central portion is preformed in the first position bowl-shape in such a way that, at the maximum pressure building up during the thermal treatment in the sealing connection zone between the flat web (6) of the annular ring (2) and the outer ring band (3a) of the panel (3) at least no substantial peeling forces are acting on the outer ring band.

7. Cover panel according to any of the preceding claims 2 to 6, wherein the angle of inclination (11) of the flat web (6) is one of greater than10°, larger than 20°, smaller than 30° or both.

8. Cover panel according to any of the preceding claims adapted for and suitable for a can with a diameter of about 83 mm, wherein an axial distance (10a) of the centre of the preformed central portion (3b), preformed in a bowl- or dome-shape, from a horizontal reference plane (15) as "the horizontal" is between 5 mm and 6 mm, in particular about 5.6 mm.

9. Cover panel of claim 2, the lid having a central portion (3b) of the lid panel (3) that is bounded by the outer ring band (3a) and deformed in a bowl-shaped fashion by preferably deep-drawing and, when doing so, having the central portion axially lowered, whereby the central portion is located several millimeters below a lowermost portion (17) of the annular ring (2).

10. Cover panel of claim 1 for providing tightness of a can closure during one of a sterilization and pasteurization of a can in a continuous autoclave, said can containing foodstuff and being closed with a lid (1) that has an annular ring (2) seamed to the can body and a lid panel (3), which is affixed with an outer ring band (3a) thereof on an inner flat web (6) that is outwardly inclined (11) with respect to a horizontal plane (15),

- wherein, prior to a seaming of the lid (1) to the filled can, a central portion (3b) of the lid panel (3) that is bounded by the outer ring band (3a) is re-shaped to an undulation-free or corrugation-free dome shape using preferably deep-drawing, the dome shape pronounced in a way that for a can with a diameter of about 83 mm, a centre of the central portion (3b) reaches an axial distance (10a) of between 5 mm and 6 mm from the horizontal plane (15,E3) assumed to extend through a transition portion (3c) between the outer ring band (3a) and the central portion (3b).

11. Cover panel of claim 2, wherein the radial flat web (6) of the annular seaming ring (2) is imparted an inclination at an angle (11), which is coordinated to an axial depth (10a, 10b) of two layers (27a, 27b; 3b, 3b') of the central portion in such a way that applying a maximal pressure during the passage of the filled and closed can through the continuous autoclave, this pressure provides no peeling effect in the sealing portion (13) of the ring band (3a) on the flat web (6) of the annular ring (2).

12. Cover panel of claim 1, wherein the central portion (3b) of the lid panel (3) is deep drawn prior to or after sealing the lid panel to the flat web (6) of the annular ring (2).

13. Cover panel of claim 2, wherein the ring band (3a) is attached to the flat web (6) by hot sealing, in particular using an induction process.

14. Cover panel of any of the prior claims, wherein the cover panel is adapted to control the pressure in a headspace (H) of a closed can body to not exceed substantially 1 bar (0.1 MPa), and this up to a sterilization temperature during sterilization.

15. Cover panel of claim 2, wherein the inclination angle (11) is between substantially 22° and 25°.

16. Cover panel of claim 1, wherein an inner tensile stress of the central portion (3b) is reduced by deep drawing said portion, in order to create a reduced stress or load of the fastening place (13) between ring band (3a) and inclined flat web (6) at a pressure load with a surface pressure on the central portion (3b) during the passage through an autoclave or retort station.

17. Cover panel of claim 1, wherein the central portion is brought into the first position shape by deep-drawing, determined by an inner space of the annular ring, radially further inside than the flat web and reaching - exposed to thermally caused surface pressure - a deflection (10b) which subjects the central portion within its material and likewise the fastening place (13) on the flat web (6) to tensile stress, said tensile stress being below a stress limit of the central portion, thus avoiding a plastic deformation, in particular avoiding tearing or cracking.

18. Cover panel of claim 2, wherein a stress in a sealing area (13) as a fastening place connecting outer ring band (3a) and inclined web (6), depends on the load or tensile stress of the central portion having a given thickness and a given differential pressure on the central portion (3b).

19. Cover panel of claim 1 or 18, wherein a stress on the central portion (3b) upon application of surface pressure is subject to a change by selecting another width of a sealing area (13) implemented as a sealing to the flat web (6).

20. A process for producing a lid closure for cans containing foodstuff, the lid providing tightness during sterilization or pasteurization in a continuous autoclave, said cans being closed with a can closure as lid (1), wherein

(i) the can closure is produced from an annular ring (2) adapted for seaming to the can body and a lid panel (3; 3a, 3b) having an outer ring band (3a), sealingly affixed onto an inner flat web (6),

(ii) a central area (3b) of the lid panel (3), limited by an outer ring band (3a), is reshaped by preferably deep drawing to a smooth bowl shape or a dome shape with a plane surface, as original shape, a material of this central area (3b) thereby solidified or hardened to such an extent that

- under an increased pressure in a headspace (H) of the can during a passage thereof through an autoclave station, the central area changes to an axially outwardly bulged shape that is mirror-inverted with respect to the original shape and, during a subsequent cooling of the can, the central area automatically returns substantially to the original shape thereof.

21. The process according to claim 20, wherein the central area (3b) of the lid panel (3) is reshaped prior to attaching the lid (1) to the filled can, preferably after a sealing of the outer ring band (3a) to the flat web (6) of the annular ring (2) or prior to a sealing to the flat web (6) of the annular ring (2).

22. The process according to claim 20 or 21, wherein the flat web (6) of the annular ring (2) is upwardly re-shaped after sealing of the lid panel (3) to the flat web (6) of the annular ring (2), the flat web still being plane at a time of sealing.

23. The process according to claim 22 or 21, wherein the reshaping, preferably by deep drawing of the lid panel and an upwards re-shaping of the flat web (6) take place substantially at the same time or the sealing takes place to an already inclined flat web (6).

24. The process of claim 20, wherein the angle of inclination (11) of the flat web (6) is one of larger than 20°, smaller than 30° or both.

25. A process of closing cans containing foodstuff that are tight during sterilization or pasteurization and such temperatures or differential pressures (ΔP) in a continuous autoclave,

(a) wherein cans are closed with can closures (1) and each closure (1) comprises a seamable annular ring (2) and a lid panel (3), the panel being affixed onto a flat web (6) by means of an outer ring band (3a) which - beforehand or thereafter - is deformed with respect to a horizontal plane (15) in an axially outwardly and upwardly inclined angle (11) in order to form an inclined flat web (6);

(b) wherein each central area (3b) of each lid panel (3) is limited by the outer ring band (3a) and deformed into a dome shape by reshaping, and a central area thereof is axially lowered to a lowermost point (30d) of the central area being several millimeters below a lowermost area (17) of said annular ring (2);

for substantially reducing a headspace (H) in each filled can upon placement of the closure (1) on the can body.

26. Process of claim 25, wherein the continuous autoclave having at least no substantial counter-pressure upon sterilizing the closed cans, preferably the counter-pressure in the continuous autoclave being below 1.6 bar (0.16 MPa).

27. Cover panel of claim 1, the continuous autoclave having no or only little counter-pressure atmosphere from an outside of the closed can.

Drawing